Negative circular polarization as a general property of n-doped self-assembled InAs / GaAs quantum dots under nonresonant optical excitation S. Laurent, 1 M. Senes, 2 O. Krebs, 1 V. K. Kalevich, 3 B. Urbaszek, 2 X. Marie, 2 T. Amand, 2 and P. Voisin 1 1 Laboratoire de Photonique et Nanostructures, CNRS, Route de Nozay, 91460 Marcoussis, France 2 Laboratoire de Nanophysique Magnétisme et Optoélectronique, INSA, 135 Avenue de Rangueil, 31077 Toulouse Cedex, France 3 A. F. Ioffe Physico-technical Institut, RAS, 194021 St. Petersburg, Russia Received 5 February 2006; revised manuscript received 12 April 2006; published 2 June 2006 We report optical orientation experiments performed under circularly polarized, nonresonant excitation in n-doped quantum dot ensembles with an average number of resident electrons ranging from zero to 4. We find a common behavior of counterpolarized luminescence, with a polarization rate building up during the recom- bination time. These results, at odds with the usual expectation, are explained in terms of interplay between periodic flip-flop mechanisms under the effect of electron-hole anisotropic exchange interaction, and irrevers- ibility due to either the thermalization or the recombination. Consequences in terms of electron-spin writing are discussed. DOI: 10.1103/PhysRevB.73.235302 PACS numbers: 72.25.Fe, 71.35.Pq, 78.67.Hc The spin properties of carriers in self-assembled quantum dots currently attract considerable attention because classical spin desorientation mechanisms acting in bulk or quantum well semiconductors should be quenched by the freezing of translational motion. Long spin depolarization time is ex- pected and actually observed 1–3 and for this reason quantum dots QDs are considered as promising candidates for the practical realization of a usable quantum bit in the technol- ogy compatible solid state environment. So far, the best un- derstood feature concerns undoped quantum dots, and the fine structure of the Coulomb-stabilized electron-hole pair 4–6 loosely termed the “exciton” in the following: the isotropic part of the electron-hole exchange  0 splits by a typical amount of 500 eV a doublet of optically inactive states or dark excitons with total angular momentum J z =±2 from a doublet of optically active states with J z =±1 or bright ex- citons. Due to anisotropy of the dot shape combined with interface anisotropy, 5 there is an anisotropic exchange split- ting AES of the bright excitons into a doublet of linearly polarized states separated by  1 =10–150 eV. The polariza- tion axis seem to be systematically along the 110 and 1 ¯ 10 axes in the case of InAs/GaAs QDs. Theory also indicates a finite splitting of the dark excitons  2  that lies in the 1 eV range and could not be measured so far. These features have been observed in both single dot and dot ensemble spectros- copy, in both the spectral 7,8 and time domains. 9–12 For in- stance, for a QD ensemble under resonant excitation linearly polarized along 110 or 1 ¯ 10, dot eigenstates are excited and a complete stability of linear polarization is observed. 10 Under circularly polarized excitation, dots are excited in a superposition of nondegenerate eigenstates and undergo pe- riodic Rabi oscillations 9,11–13 corresponding to a periodic flip-flop of electron and hole spins with period 2 ff = h /  1 . However, as a result of strong dispersion of  1 values in the dot ensemble or of pure dephasing mechanisms in time- integrated measurements, a rapid dephasing of these quan- tum beats and a fast decay of circular polarization are observed. 9,12,13 For the same reasons, a very weak circular polarization is observed under nonresonant excitation. All these results are explained satisfactorily by consideration of the AES. A single point has not been elucidated in full detail: this is the dynamics of dark excitons. At this moment, a likely assumption is that, at low temperatures, dark excitons disappear through a nonradiative mechanism rather than through a spin-flip mechanism transforming them into bright excitons. 14 We note that in absence of such a nonradiative process, dark excitons would contribute to a luminescence signal with a long decay time as reported for CdSe quantum dots 15 . The time-integrated luminescences associated with excitons created, respectively, in the dark and bright states would be equal. No systematic observation of such slow de- cay has been reported so far in InAs quantum dots, and we do not observe a “dark exciton” line in cw single-dot spec- troscopy. The observation of linear polarization stability of bright excitons excludes a fast process transforming dark states into bright states and vice versa. Again, the relax- ation process of dark excitons is an important topic that de- serves more investigations. Here, we focus on spin dynamics in n-doped quantum dots. This situation is far more complicated, in particular because of the strong exchange interaction of resident and photoinjected electrons or holes leading to effects such as Pauli blocking of the thermalization when resident and pho- toinjected carriers have parallel spins. 16 Also, in doped samples, a significant dispersion of the number of resident carriers is expected from dot to dot, or as a function of time in the presence of optical excitation. A totally unusual nega- tive rate of circular polarization with remarkable dynamics was first reported by Cortez et al., 2 together with a long- duration 15 ns optically driven spin-memory effect, for a sample having nominally a single resident electron per dot. The mechanism of spin writing was tentatively described in terms of interplay between periodic electron-hole flip-flops due to AES and irreversible thermalization of Pauli- blocked electron-pair configurations. The memory effect was explained in terms of the single resident-electron spin po- larization lifetime. Here, we extend such investigations to a series of samples of different origin and having nominal dop- ing levels from zero up to more than four electrons per dot, and find that the negative circular polarization is a common PHYSICAL REVIEW B 73, 235302 2006 1098-0121/2006/7323/2353027 ©2006 The American Physical Society 235302-1